Plant Nutrients and Nutrient Management
The seventeen essential elements
Carbon, hydrogen and oxygen come from air and water. The remaining fourteen are absorbed from soil: nitrogen, phosphorus, potassium, calcium, magnesium and sulphur (macronutrients, needed in larger amounts) and iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel (micronutrients). Sodium, silicon and cobalt are beneficial but not universally essential.
Functions and mobility
- N: proteins, chlorophyll, nucleic acids; P: ATP, phospholipids, root development; K: enzyme activation, stomatal regulation, grain quality; Ca: calcium pectate of cell walls; Mg: central atom of chlorophyll; S: cysteine and methionine.
- Fe: chlorophyll synthesis; Mn: water splitting in photosystem II; Zn: tryptophan (auxin) synthesis; B: pollen germination and cell wall; Mo: nitrogenase and nitrate reductase; Ni: urease.
- Mobile nutrients (N, P, K, Mg) move from old to young tissue, so deficiency appears first in older leaves. Immobile nutrients (Ca, Fe, Mn, Zn, Cu, B) show deficiency in young leaves and growing points.
Worked example
In citrus on a calcareous soil (pH 8.2), young leaves show interveinal chlorosis with green veins, while old leaves are normal. Reasoning: young leaves mean an immobile nutrient; interveinal chlorosis narrows it to Fe, Mn or Zn; Zn usually gives small "rosette" leaves and Mn gives necrotic spots, so iron deficiency is diagnosed - iron is insoluble at high pH. Management: soil application is ineffective; use ferrous sulphate sprays plus organic matter.
Nutrient management rules
Soil-test based recommendations, the 4R principle (right source, rate, time, place), and integrated nutrient management combining organic manure, mineral fertilizer and biofertilizer. Watch antagonism: excess K depresses Ca and Mg uptake; excess P induces Zn deficiency.
Traps
Placing N deficiency in young leaves; calling boron mobile; assuming more fertilizer always means more yield (diminishing returns, lodging, toxicity).